early giraffe relative at the bottom and modern giraffes at top
early giraffe relative at the bottom and modern giraffes at top

“Necks for Sex” May Explain Giraffes’ Distinctive Anatomy 

An analysis of skull and vertebrae fossils suggests that an early relative of giraffes butted heads to compete for mates, which may reveal why modern giraffes are so throaty.

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Andy Carstens

Andy Carstens is an intern at The Scientist. He has a bachelor's degree in chemical engineering from the Georgia Institute of Technology and a master's in science writing from Johns Hopkins University.

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Jun 3, 2022

ABOVE: An early giraffe relative, Discokeryx xiezhi WANG Yu and GUO Xiaocong

Students often learn that giraffes (Giraffa spp.) evolved their sinuous necks to eat high-dangling leaves that stumpier competitors couldn’t quite reach. However, thickened skull and vertebrae fossils of the early giraffe relative Discokeryx xiezhi indicate it headbutted rivals to win mates, which may help explain why today’s giraffes evolved such stretchy necks, a study published yesterday (June 2) in Science finds.

Researchers found the giraffoid’s skull and four cervical vertebrae fossils, which date to about 17 million years ago, in 1996 in the Junggar Basin of northern China, reports Nature. Analyzing the fossils revealed a thickened cranium with a disk-shaped structure that acted like headgear, along with a complicated array of head-and-neck joints and similarly-thickened vertebrae, all of which made D. xiezhi well-suited for cranial combat, according to the study.

Discokeryx has extreme morphologies of the head and neck adapted for head-butting behavior,” study coauthor and paleontologist Jin Meng tells Reuters.

The long necks of modern giraffes should provide enough benefit to counter the energetic cost of pumping blood through a six-foot riser to reach their brains, according to Nature. For decades, the prevailing theory has been that modern giraffes evolved long necks to reach higher food sources. “It’s beautifully adapted to this, but it’s a big cost,” Rob Simmons, a behavioral ecologist at the University of Cape Town in South Africa who was not involved in the study, tells Nature. The eating hypothesis is far from certain, given that giraffes often feed on low food sources, and that longer necks aren’t associated with increased survival during periods of drought when food is scarcer, reports Nature.

Another explanation that hasn’t gained as much traction among giraffe researchers is the “necks for sex” hypothesis, according to Nature. This theory postulates their elongated necks arose from mating competitions, such as those between modern male giraffes, which bang their necks together in a violent display that’s often won by the longer-necked rival, according to Reuters. Because the ancient head-to-head battles of D. xiezhi could be related to modern giraffe neck fights, the researchers tell Reuters that their findings give a glimpse into how neck elongation may have begun evolving millions of years ago.

But not everyone agrees. “All ruminants fight with their horns and neck,” Nikos Solounias, a paleoungulate biologist at the New York Institute of Technology College of Osteopathic Medicine tells Nature. D. xiezhi may not be that close of a relative to modern giraffes, he argues, so their headbutting behavior may not tell us much about how long necks evolved. 

On the other hand, Simmons tells Nature he believes the new findings indicate the “necks for sex” idea is as likely to be true as the eating hypothesis. “This paper is going to open people’s eyes that we should take this sexual selection theory seriously,” he says.